Optical communication systems transmit light signals over distances ranging from less than a few meters to over hundreds of kilometers. As the light signals propagate in optical waveguides they become distorted in various ways because the waveguides are not an ideal transmission media.
Dispersion is one of the most prevalent forms of distortion in optical transmission systems. Dispersion causes the transmitted optical waveforms to undesirably change shape as the various wavelength components that make up the signals propagate at slightly different speeds through the waveguide.
Optical components can be used to correct various types of signal distortion caused by transmission through waveguides. Some of the most commonly used devices are dispersion compensators and dispersion slope compensators.
Tunable dispersion compensators can be tuned to effect correction at specific wavelengths. Tunable dispersion compensators have been realized using optical allpass filters implemented with ring resonators in planar waveguides and Gires-Tournois cavities using thermal and micro-mechanical tuning. Variable optical delay lines and dispersion slope compensators can also be realized with the same filter structures. Other applications for variable delays include polarization mode dispersion compensation, optical buffering and other optical signal processing.
For allpass filters, there is an inherent tradeoff between dispersion and filter bandwidth. That is, dispersion is increased in a single allpass filter stage only at the cost of a concomitant decrease in bandwidth. However, by increasing the number of stages, the dispersion can be increased for a given bandwidth. The stages are optically coupled, or cascaded in series.
Cascading creates many tradeoffs, including the ability to make tradeoffs between dispersion, bandwidth, and approximation error (or accuracy). Cascading, however, tends to produce undesirable features in the filter spectral response, including amplitude ripple over the desired filter bandwidth caused by lossy filters.
What is needed is a new optical filter architecture that exhibits better tradeoffs between the phase and amplitude characteristics and ease of fabrication than cascaded filters over a desired filter bandwidth.